Hi chris7!
Besides a good medical history, a clinical assessment is necessary in every case, including yours. Without both, any assumption might be wrong. I'll try, though, to make some guesses.
If I could hypothesize something, mainly from the type of injury, I would say that a muscle strain at the left leg was probably an initial injury. You didn't mention where the pain was located but my guess is at the hamstrings, as they tend to strain quite often. The fact that pain was gone at the left and appeared at the right leg makes me wonder about low back involvement. You also said that pain goes off and on at both legs and that location differs (from mid hamstrings to your gluteus etc). Taking these facts into account, low back should definitely be examined. Initial injury might be a fact but it's not a strain that causes such problems after 4-5 months.
My advice is to visit a physio, have a clinical assessment (I assume he/she will assess both legs and the low back) and then we can discuss about treatment. Remember that this is only a guess, actually my first one. Other pathologies or anatomical alterations/abnormalities could also be the case. Moreover, more details about the history are necessary (type of pain, any weaknesses, any numbness, positions that aggravate/eliminate symptoms etc).
Looking forward to hearing from you.
ilias